One of the broad aims of research in perception is to find out how humans make sense out of the infinitely varying flux reaching the retina. It is now common among perceptual psychologists to view perception as a series of transformation and operations on stimulus input. In broad outline, making sense out of the visual input can be divided into the sequence: registration, description, and interpretation. All three categories are thought to involve various kinds of transformations on the initial stimulus; in particular, registration and description are thought to involve the abstraction of properties or features of the stimulus. The interpretive operations are thought to occur within a framework of internal models of how the world is supposed to look. Due to difficulties involved in finding behavioral measures of operations "inside the head" most of the operations in the pattern recognition sequence are unknown. But behavioral measures are available: visual masking techniques. These techniques can provide insights into visual system spatial and temporal organization; and, since there is evidence that the visual system is active throughout the process of pattern recognition and does not cease responding after the registration of stimuli, these same measures may be able to characterize higher order operations in the pattern recognition sequence as well. The overall research strategy of this project then is to utilize visual masking techniques as a means of investigation, for that part of the pattern recognition sequence which involves the visual system, how this sequence occurs, and what, specifically, is involved in registration, description, and interpretation of a stimulus.